S329
Clinical - Breast
ESTRO 2026
radiotherapy and conservative surgery ranged from 4 weeks to 8 months. The random-effects meta-analysis estimated a pooled pathological complete response (pCR) of 13% (95% CI: 2–30%), with high heterogeneity across studies (I ² = 80.8%, p < 0.001)(Figure 1).
Digital Poster Highlight 4554 Pathological response in pre-operative stereotactic radiotherapy in early-stage breast cancer: systematic review and meta-analysis Giuseppe Facondo 1 , Yvonne Beorchia 2 , Agnese Prisco 1 , Chiara Reverberi 1 , Eleonora Festa 1 , Luigi Castriotta 2 , Marco Trovo 1 1 Radiation Oncology, Santa Maria della Misericordia, Udine, Italy. 2 Institute of Hygiene and Evaluative Epidemiology, Santa Maria della Misericordia, Udine, Italy Purpose/Objective: to analyze the pathological response in patients with early breast cancer undergoing preoperative stereotactic radiotherapy. This topic is becoming increasingly relevant, as promising responses have been reported in the literature. Material/Methods: The systematic review was registered with the International Prospective Register of Systematic Reviews (PROSPERO CRD420251132534). We searched five databases, namely PubMed, Scopus, Web of Science, Cochrane, ScienceDirect and ClinicalTrials.gov registry. An additional evaluation was conducted via citation searching from selected articles. Two blinded authors independently performed searches.We included all randomized controlled trials, longitudinal observational studies, case–control studies, and retrospective and prospective cohort studies that investigated preoperative stereotactic radiotherapy in early-stage breast cancer. Risk of bias in randomized controlled trials was assed using The Cochrane Collaboration's tool for assessing risk of bias in randomized trials. Risk of bias of non-randomized studies of interventions will be assessed using the ROBINS-I V2 tool. A random-effects meta-analysis of proportions was conducted using the Freeman–Tukey double arcsine transformation and the DerSimonian and Laird method. Confidence intervals were calculated using the Wilson score method. Meta- analysis was performed using STATA software (Stata 14, Stata Corporation, Texas). Results: Of the 1956 results, 38 duplicates were removed, 1939 were excluded, 13 were included from other references. Of the 30 selected, only 5 met the eligibility criteria. All the selected studies were prospective phase I or II trials. The median number of patients enrolled in the studies was 22 (IQR 16-42). Only early- stage (cT1-2 cN0) and patients over 50 years of age were selected. The median dose administered was 24 Gy (range 15-36) delivered in 1–3 fractions. Four out of five studies implanted fiducial markers in the tumor. The pathological complete response (pCR) across all studies ranged from 0 to 41.6%. The interval between
Conclusion: The results of this systematic review and meta-analysis although with limitations, suggest that a higher total dose delivered and the time interval between radiotherapy and surgery may increase the probability of achieving a major response including pCR. There are several limitations: all included studies are observational cohort studies, and RCTs are lacking to date. Also, there is heterogeneity across these studies. Keywords: Pathological Response, pre-operative breast cancer Digital Poster Highlight 4598 Cost-Effectiveness Analysis of Ultrahypofractionated vs Moderately Hypofractionated Whole Breast Radiotherapy: A Canadian Perspective Ammar Salkini 1,2 , Hayeon Kim 3 , Shirley Tse 4 , Michael Lock 5 , Gustavo N Marta 6,7 , Jeffrey Q Cao 8,9 , Iwa Kong 10,11 , Tarek Hijal 12,13 , Justin Lee 14 , Irene Karam 2,15 , Edward Chow 2,15 , Shing Fung Lee 16,17 1 MD Program, Temerty Faculty of Medicine, University of Toronto, Toronto, ON, Canada. 2 Department of Radiation Oncology, Temerty Faculty of Medicine, University of Toronto, Toronto, ON, Canada. 3 Department of Radiation Oncology, Magee-Womens Hospital, UPMC Hillman Cancer Center, Pittsburgh, USA. 4 Department of Clinical Oncology, United Christian Hospital, Kowloon East Cluster, Hospital Authority,, Hong Kong S.A.R, China. 5 Department of Oncology, London Health Sciences Centre, Western University, London, ON, Canada. 6 Department of Radiation Oncology, Hospital Sírio-Libanês, São Paulo, Brazil. 7 Breast Cancer Group, Latin American Cooperative Oncology Group (LACOG), Porto Alegre, Brazil. 8 Division of Radiation Oncology, Arthur J.E. Child Comprehensive Cancer Centre, Calgary, AB, Canada. 9 Department of Oncology, University of Calgary,
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